1. Field of the Invention
The present invention relates to an optical integrated circuit substrate having an optical waveguide and a thin-film optical element which are integrated on the same substrate, more particularly to an optical integrated circuit substrate which is, like a WDM (Wavelength Division Multiplex) optical module substrate, suitably used for the case where a plurality of thin-film optical elements and other devices need to be mounted on the same substrate and in which miniaturization of the substrate, improvement of productivity, and enhancement of optical transmitting/receiving efficiency are achieved by integrating together an optical waveguide and a thin-film optical element on the same substrate.
The invention also relates to an optical module, which is used for an optical signal transmission system, having a semiconductor light-emitting element, an optical waveguide, and a monitoring semiconductor light-receiving element.
2. Description of the Related Art
In recent years, research and development have been under way on an optical element which lends itself to improvement of capability and productivity of an optical transmission module, and also on a technique for mounting an optical element with higher density, higher accuracy, and higher optical connection efficiency.
For example, xe2x80x9cThin-Film Multimaterial Optoelectronic Integrated Circuitsxe2x80x9d carried in xe2x80x9cIEEE Transactions on Components, Packaging, and Manufacturing Technology, part B, Vol. 19, No. 1, February 1996xe2x80x9d deals with a technique whereby an optical light-receiving element is grown epitaxially on a semiconductor substrate and thereafter only the resultant epitaxial layer is isolated therefrom so as to form a thin-film optical light-receiving element to be mounted on another mounting substrate. According to this technique, a thin-film optical element made of various materials can be mounted on a mounting substrate with higher density and higher accuracy.
Moreover, as an example of optical element mounting techniques, an optical integrated circuit substrate proposed in Japanese Unexamined Patent Publication JP-A 7-128531 (1995) is shown in section in FIG. 8. In FIG. 8, the optical integrated circuit substrate includes: a substrate 31; an optical waveguide 32 having a lower clad layer 34, a core layer 35, and an upper clad layer 36; and a surface light-receiving element 37, built as a thin-film optical element, disposed on the substrate 31 such that its light-receiving surface is covered with the lower clad layer 34. In this construction, an electromagnetic field of light propagating around the core layer 35 is spread out over the lower clad layer 34. This makes possible optical connection with the surface light-receiving element 37.
On the other hand, in the case where optical connection is established between the optical waveguide formed on the substrate and the thin-film optical element embedded in the optical waveguide, the following problem arises.
The thin-film optical element is composed solely of an epitaxial layer and thus has a thickness of no greater than several xcexcm. Moreover, in a typical single-mode optical waveguide, difference in specific refractive index between the cladding and the core falls in a range of 0.2 to 1.5%, and the core has a thickness of about 4 to 8 xcexcm. Here, to minimize the interaction between the substrate and light to be transmitted, the thickness of the lower cladding needs to be made more than 1.5 times as large as that of the core, more specifically, the lower cladding needs to have a thickness of about 6 to 12 xcexcm. Meanwhile, to bring sufficiently high efficiency to the optical connection between the optical waveguide and the thin-film optical element arranged therebelow, the lower cladding of the optical waveguide needs to be made thin enough to reduce the distance between the core and the thin-film optical element.
Conventionally, after a thin-film optical element is formed or arranged on a substrate surface, an optical waveguide is formed thereon by coating. Accordingly, in the case where, after the thin-film optical element 37 is disposed on the substrate 31, an optical waveguide is formed thereon so as to achieve optical connection, as shown in FIG. 8, it is necessary to provide a curve or bend portion 38 in the optical waveguide 32 so that, of the lower clad layer 34, one part located above the thin-film optical element 37 is made thin, and the other part free of the thin-film optical element 37 is made thick. In this case, if the curve portion 38 has an unduly large curvature, the interaction between the substrate 31 and transmitted light occurs over a wider area in the vicinity of the thin-film optical element 37, which results in significant light transmission losses. By contrast, if the curve portion 38 has an unduly small curvature, although the interaction between the substrate 31 and transmitted light is prevented from occurring over a wide area in the vicinity of the thin-film optical element 37, transmitted light radiates over the curve portion 38, which results in significant light transmission losses and occurrence of stray light which causes cross talk.
JP-A 7-128531 further proposes, as Practical example 3, a construction fabricated in the following manner. A semiconductor layer of substantial height is used as a base substrate so that an active layer or a light absorbing layer acting as a thin-film optical element is located at a considerable distance from the substrate. Then, an optical waveguide is formed thereon by coating. In this construction, however, the core of the optical waveguide is significantly bent in the vicinity of the optical element. This causes radiation losses of light in the bend portion and also causes scattering losses of light in the optical element portion. Another problem with this construction is that, in forming an optical waveguide by coating, because of stepped configuration created due to the arrangement of the optical element, the process accuracy of the core of the optical waveguide is deteriorated, or it is difficult to form the coated or bend portion around the optical element into desired shape. This makes it impossible to obtain satisfactory performance capability as intended.
Moreover, for an optical signal transmission system, an optical module is used that includes: an optical waveguide formed on a substrate; a semiconductor light-emitting element arranged so as to be optically connected to the optical waveguide; and a monitoring semiconductor light-receiving element for detecting intensity of light which is emitted from the semiconductor light-emitting element and transmitted through the optical waveguide. The monitoring semiconductor light-receiving element serves to stabilize optical output from the semiconductor light-emitting element by monitoring the intensity of the light emitted from the semiconductor light-emitting element and then providing feedback for a driving circuit of the semiconductor light-emitting element.
Shown in FIG. 9 as a plane figure is a conventional optical module proposed in Japanese Unexamined Patent Publication JP-A 11-38279 (1999).
In the optical module shown in FIG. 9, on a substrate 41 is formed an optical waveguide 46 and mounted a semiconductor light-emitting element 47 (a laser diode is used here). The laser diode has two excitation ends composed of semiconductor cleavage planes. In addition, on the substrate 41 is mounted a monitoring semiconductor light-receiving element 42 opposed to one of the excitation ends. In this construction, backward light emitted from the semiconductor light-emitting element 47 (light to be monitored) is monitored by the semiconductor light-receiving element 42, and the output of the semiconductor light-emitting element 47 is so controlled as to be kept constant by an optical output level stabilizing circuit (not shown).
However, the above-described conventional optical module having the semiconductor light-emitting element 47, the optical waveguide 46, and the monitoring semiconductor light-receiving element 42 has the following various disadvantages.
First, in the case where an edge-emitting type laser diode is used as the semiconductor light-emitting element 47 and backward light emitted therefrom is monitored by the semiconductor light-receiving element 42, the optical output ratio between the forward and backward light emitted from the semiconductor light-emitting element 47 is not necessarily kept constant. Therefore, backward light output is detected by indirectly monitoring forward light output, which may result in inaccurate detection. To detect backward light output with accuracy by directly monitoring forward light output, forward emitted light should preferably be monitored.
To achieve this, as shown in FIG. 10 in section, Japanese Unexamined Patent Publication JP-A 8-330661 (1996) proposes an optical module employing a surface-emitting laser, wherein a beam splitter is inserted into an optical system through which forward emitted light in use travels and part of the forward emitted light is split by the beam splitter to create monitoring light.
In the optical module shown in FIG. 10, a beam splitter 58 is fixed to a light-emitting surface of a surface-emitting type semiconductor light-emitting element 57, and part of the laser beam emitted from the semiconductor light-emitting element 57 is split by the splitter to create monitoring light. Then, the monitoring light is detected by a semiconductor light-receiving element 52.
In this case, however, the beam splitter 58 is additionally required to split forward light for monitoring output, and this leads to an undesirable increase in the number of components or assembly process steps for constituting the optical module. This makes positioning for the optical system complicated and also makes the size of the optical module unduly large.
Moreover, assume that forward light emitted from a surface-emitting type semiconductor light-emitting element is monitored. In this case, unlike an edge-emitting type semiconductor light-emitting element, light is emitted only from one side of the surface-emitting type light-emitting element. Thus, to secure a path for directing light to a monitoring semiconductor light-receiving element, a beam splitter or the like needs to be arranged partway along a light transmission path, such as an optical waveguide or an optical fiber, to split an optical path. This complicates the structure or assembly process of the optical module and makes miniaturization of the optical module difficult.
An object of the invention is to provide an optical integrated circuit substrate capable of achieving low-loss light transmission and establishing satisfactory optical connection between a thin-film optical element arranged on the substrate and an optical waveguide formed thereon by coating.
Another object of the invention is to provide an optical integrated circuit substrate capable of achieving low-loss light transmission and establishing satisfactory optical connection between a thin-film optical element arranged on the substrate and an optical waveguide formed thereon by coating, and also provide an improved optical integrated circuit substrate which is impervious to being affected by external noise and in which reflection and losses of high-frequency signals inputted/outputted from a wiring conductor to the thin-film optical element are reduced.
Still another object of the invention is to provide an optical module having a semiconductor light-emitting element, an optical waveguide, and a monitoring semiconductor light-receiving element, which is capable of monitoring forward light emitted from the semiconductor light-emitting element with accuracy, and can be realized with higher productivity in a simple construction requiring fewer components.
The invention provides an optical integrated circuit substrate comprising:
a substrate;
an optical waveguide formed on the substrate, the optical waveguide having a cladding and a core;
a metal placement portion formed on the substrate; and
a thin-film optical element placed on the metal placement portion,
wherein the metal placement portion and the thin-film optical element are embedded in the optical waveguide.
According to the optical integrated circuit substrate embodying the invention, on the substrate is formed an optical waveguide having a cladding and a core. Within the optical waveguide is formed a metal placement portion for placing an optical element. A thin-film optical element is placed on the metal placement portion. Since the metal placement portion and the thin-film optical element are embedded in the optical waveguide, by reducing a distance between the thin-film optical element and the core of the optical waveguide, or by arranging the thin-film optical element within the core, optical signal transfer can be efficiently achieved between the optical element and the core. Moreover, in a region of the optical waveguide free of the thin-film optical element, an adequate distance can be secured between the core of the optical waveguide and the substrate. This makes it possible to establish satisfactory optical connection between the thin-film optical element embedded in the optical waveguide and the optical waveguide, and to achieve low-loss light transmission without being affected by the interaction between light transmitted through the optical waveguide and the substrate.
Moreover, the thin-film optical element is placed on the metal placement portion before it is embedded in the optical waveguide. Therefore, by dint of the metal placement portion, signal input and output, power supply, and heat conduction and dissipation are performed directly on the thin-film optical element with efficiency. This makes it possible to perform input and output of high-frequency signals and high-power optical signals with stability. As a consequence, a high-performance, highly-reliable optical signal operation can be realized that is excellent in high-frequency characteristics and operation stability.
Further, both of the metal placement portion and the thin-film optical element to be embedded in the optical waveguide can be fabricated in a thin-film forming process similar to an optical waveguide manufacturing process. This is advantageous in terms of high processing accuracy, high-density arrangement, and excellent productivity.
The invention further provides an optical integrated circuit substrate comprising:
a substrate;
an optical waveguide formed on the substrate, the optical waveguide having a cladding and a core;
a coplanar line including a line conductor dividedly extending on the substrate and grounding conductors formed on both sides of the line conductor so as to extend in spaced parallel relation to each other;
a metal electrode arranged differently in level than the line conductor, the metal electrode being electrically connected to the line conductor so as to bring the divided line conductor into conduction; and
a thin-film optical element electrically connected to the metal electrode,
wherein the metal electrode and the thin-film optical element are embedded in the optical waveguide,
and wherein a horizontal distance between the metal electrode and the grounding conductor is made smaller than a horizontal distance between the line conductor and the grounding conductor.
According to the invention, in an optical integrated circuit, on a substrate is formed an optical waveguide having a cladding and a core, and a metal electrode and a thin-film optical element electrically connected to the metal electrode are embedded in the optical waveguide. In this construction, by reducing a distance between the thin-film optical element and the core of the optical waveguide, or by arranging the thin-film optical element within the core, optical signal transfer can be efficiently achieved between the optical element and the core. Moreover, in a region of the optical waveguide free of the thin-film optical element, an adequate distance can be secured between the core of the optical waveguide and the substrate. This makes it possible to establish satisfactory optical connection between the thin-film optical element embedded in the optical waveguide and the optical waveguide, and to achieve low-loss light transmission without being affected by an interaction between light transmitted through the optical waveguide and the substrate.
Moreover, the thin-film optical element is electrically connected to the metal electrode before it is embedded in the optical waveguide. Therefore, by dint of the metal electrode, signal input and output, power supply, and heat conduction and dissipation are performed directly on the thin-film optical element with efficiency. This makes it possible to perform input and output of high-frequency signals and high-power optical signals with stability. As a consequence, a high-performance, highly-reliable optical signal operation can be realized that is excellent in high-frequency characteristics and operation stability.
Further, both of the metal electrode and the thin-film optical element to be embedded in the optical waveguide can be fabricated in a thin-film forming process similar to an optical wave guide manufacturing process. This is advantageous in terms of high processing accuracy, high-density arrangement, and excellent productivity.
In the optical integrated circuit embodying the invention, the metal electrode and the divided line conductor of the coplanar line, being in existence at different levels, are electrically connected to each other. Moreover, the grounding conductors of the coplanar line are formed on both sides of the line conductor including a divided portion so as to extend in spaced parallel relation to each other. Further, a horizontal distance between the metal electrode and the extendedly-formed grounding conductor is made smaller than a horizontal distance between the line conductor of the coplanar line and the grounding conductor. This makes it possible to reduce the difference in impedance between the coplanar line and the metal electrode, and further to match the characteristic impedance of the former to that of the latter. Consequently, it is possible to achieve about 0.1 to 20% reduction in reflection and losses of high-frequency signals inputted or outputted to the thin-film optical element by the coplanar line, as well as about 0.1 to 20% reduction in external noise.
As described thus far, according to the invention, there are provided an optical integrated circuit substrate capable of achieving low-loss light transmission and establishing satisfactory optical connection between a thin-film optical element arranged on a substrate and an optical waveguide formed thereon by coating, and an improved optical integrated circuit substrate which is impervious to being affected by external noise, wherein reflection and losses of high-frequency signals inputted or outputted from a wiring conductor to a thin-film optical element are reduced.
In the invention, it is preferable that the optical waveguide is made of a siloxane polymer, fluorinated polyimide, fluorine resin, polymethylmethaacrylate (PMMA), or polycarbonate (PC).
In the invention, it is preferable that the metal placement portion is made of Au, Ti, Pd, Pt, Al, Cu, W, or Cr.
In the invention, it is preferable that on an outer surface of the metal placement portion is formed a soldered layer made of AuSn or AuGe.
In the invention, it is preferable that the thin-film optical element is built as a pn photodiode, a pin photodiode, a phototransistor, an MSM (Metal-Semiconductor-Metal) photodiode, an avalanche photodiode, a light-emitting diode, a vertical resonator type surface emitting laser, or an edge emitting laser.
The invention still further provides an optical module comprising:
a substrate;
an optical waveguide formed on the substrate, the optical waveguide having a cladding and a core;
a semiconductor light-emitting element arranged on the substrate so as to be optically connected to the optical waveguide; and
a semiconductor light-receiving element arranged on the substrate so as to be optically connected to the optical waveguide, the semiconductor light-receiving element detecting light to be transmitted through the optical waveguide, the transmitted light being emitted from the semiconductor light-emitting element,
wherein the semiconductor light-receiving element is disposed in a vicinity of the core with its light-receiving surface arranged parallel to a surface of the substrate so as to receive a transmitted light leak from the core of the optical waveguide.
According to the invention, an optical module comprises: a substrate having an optical waveguide formed thereon that includes a cladding and a core disposed inside the cladding; a semiconductor light-emitting element arranged on the substrate so as to be optically connected to the optical waveguide; and a semiconductor light-receiving element arranged on the substrate so as to be optically connected to the optical waveguide, the semiconductor light-receiving element detecting intensity, for example, of light to be transmitted through the optical waveguide, the transmitted light being emitted from the semiconductor light-emitting element. The semiconductor light-receiving element is disposed in the vicinity of the core with its light-receiving surface arranged parallel to the surface of the substrate so as to receive an evanescent wave of a leakage component of the transmitted light from the core of the optical waveguide. Accordingly, there is provided an optical module having a semiconductor light-emitting element, an optical waveguide, and a monitoring semiconductor light-receiving element, which is capable of monitoring forward light emitted from the semiconductor light-emitting element with accuracy without using an optical splitting element, such as a beam splitter, for splitting monitoring light for the forward emitted light, and can be realized with higher productivity in a simple construction requiring fewer components.
In the invention, it is preferable that the optical waveguide is made of a siloxane polymer, fluorinated polyimide, fluorine resin, polymethylmethaacrylate (PMMA), or polycarbonate (PC).
In the invention, it is preferable that the semiconductor light-receiving element is built as a pn photodiode, a pin photodiode, a phototransistor, an MSM (Metal-Semiconductor-Metal) photodiode, or an avalanche photodiode.
In the invention, it is preferable that the semiconductor light-emitting element is built as a laser diode, alight-emitting diode, a vertical resonator type surface emitting laser (VICSEL), or an edge emitting laser.